1. Field of the Invention
The present invention relates to a photomask blank for a photomask for use in fabrication of a semiconductor integrated circuit and the like and a method for manufacturing a photomask blank.
2. Description of the Related Art
In recent semiconductor processes, particularly the increasing scale of integration of large-scale integrated circuits increasingly requires the shrinking of circuit patterns. There is a growing demand for shrinking techniques of wiring patterns formed in circuits or contact hole patterns for wiring between layers formed in cells. In production of photomasks, in which circuit patterns are formed, for use in optical photolithography for forming these wiring patterns or contact hole patterns, there is accordingly a need for a technique to precisely form finer circuit patterns due to the above shrinking of the patterns.
Forming a resist pattern with high precision on a photomask blank is first needed to form a photomask pattern with higher precision on a photomask substrate. When an actual semiconductor substrate is processed, since optical photolithography employs reduced-size projection, the photomask pattern needs a size that is about four times greater than an actually needed pattern size. This however does not mean that the required precision is accordingly lowered, but a higher precision than that of the pattern after exposure is rather needed for a photomask serving as an original.
In addition, photolithography currently employed draws a circuit pattern with a significantly lower size compared with a wavelength of light to be used. Accordingly, if a photomask pattern having a size four times greater than that of a circuit form is used, then the photomask pattern form is not accurately transferred to a resist film due to influence of light interference occurring when the optical photolithography is performed, etc. For the purpose of reducing the influence, the photomask pattern sometimes needs to be formed into a more complex form (a form to which optical proximity correction (the so-called OPC) is applied) than an actual circuit pattern. There is accordingly a need for a higher-precision processing method in the photolithography technique to obtain photomask patterns. The performance of lithography may be represented by a resolution limit. The lithography technique in photomask forming processes requires a resolution limit substantially identical to or more than the resolution limit needed in optical photolithography employed in semiconductor processes using photomasks.
The procedure for forming a photomask pattern typically involves forming a photoresist film on a photomask blank having a light-shielding film on a transparent substrate, drawing a pattern by an electron beam, obtaining a resist pattern through development, and then etching the light-shielding film to form a light-shielding pattern while using the obtained resist pattern as an etching mask. If it attempts to achieve the shrinking of the light-shielding pattern while the same thickness of the resist film as that before the shrinking is maintained, then a ratio of a film thickness to the pattern, i.e., the an aspect ratio, becomes larger and pattern transfer thereby fails due to deterioration of the pattern form of the resist, or the resist pattern is broken or separated in some cases. The shrinking accordingly necessitates thinning a resist film thickness.
Use of hard masks has been tried before to reduce a burden on resists during dry etching. For example, Patent Document 1 reports that an SiO2 film formed on MoSi2 is used as an etching mask when MoSi2 is dry etched with a gas containing chlorine, and the SiO2 film can also function as an antireflection coating. In addition, there is disclosed that chromium is used for a light-shielding film on a phase shift film and an SiO2 film on the light-shielding film is used as a hard mask in, for example, Patent Document 2.